Passive Radio-Frequency Identification (RFID) tags typically consist of an integrated circuit (IC) connected to an antenna. The IC is often a low power device, powered purely by RF energy harvested from the reader signal. The tag responds to the reader by varying its input impedance (and reflectance) and thus modulating the backscattered signal.
In RFID systems, both forward (reader-to-tag) and reverse (tag-to-reader) links are important. Current passive CMOS RFID ICs are approaching the fundamental limits of their turn-on sensitivity, dictated by diode-based voltage multiplier limits of RF-to-DC power conversion. The best ICs currently have sensitivity of about −20 dBm which has been reached for several years and no further sensitivity improvement is expected.
Often the overall system performance is limited by the tag sensitivity; in other words, the tag is the weakest link in the communication system. For many applications, such as for tag reading in indoor multi-tag multipath scenarios, or AVI tolling applications, better passive tag sensitivity is desired. Every dB of improvement in tag sensitivity results in measurable system performance improvement, e.g. in the useable range of the tag.
In the past, improvements to tag sensitivity and range have been attempted. One such solution proposes a tag which can combine voltages from two ports of orthogonal dipole antennas to gain more tag sensitivity but requires a large cross-dipole tag and the presence of circularly polarized reader signal in order to extract power from both polarizations. Another solution is to eschew a passive tag design for a powered tag design, or to sacrifice bandwidth. These solutions increase the size of the tag and/or require battery replacement, often rendering them unsuitable for their target applications. Sacrificing bandwidth is also unacceptable in many applications.
Traditionally, the tag transmission performance has been viewed as paramount. The prevailing belief is that a reduction in tag transmission performance would reduce the range of the tag. Thus, solutions that improve tag range by sacrificing tag transmission performance have not been explored.